Symptomatic adult and pediatric structural heart disease affects more than 2.9% of the US population, not including cardiomyopathies and rhythm disorders. Although nonsurgical approach is the preferred route for relief of symptoms, even common procedures such as atrial septal defect closure can be difficult, protracted, or unsuccessful because of limitations not only in available catheter devices but also because of inadequate image guidance. Current 2D and limited-volume 3D intracardiac ultrasound catheters do not provide suitable full-volume en face images to depict complex cardiac structures in real time, do not depict real-time navigation of catheters, and require adjunctive X-ray guidance. Miniaturization of ultrasound probes to provide uninterrupted real-time full-volume intra-procedural 3-dimensional en-face depiction of cardiac pathology and catheter devices would represent a dramatic advance in image-guided intervention. Further, design of a 3D ICE probe from inception to operate safely during MRI as well as during conventional X-ray would dramatically advance or even revolutionize the capabilities of transcatheter therapy by enabling completely radiation-free non-surgical catheter navigation. Our proposal aims to realize such a catheter, the 3D MRICE, based on the close collaboration of two groups at NHLBI and at Georgia Tech, and the unique resources available at the NIH Clinical Center for catheter fabrication, preclinical evaluation, and clinical trials. We use CMUT-on-CMOS technology and massive parallel RF data multiplexing to implement an ultra- miniature, single chip volumetric ultrasound imaging system while reducing the transmission line count by 50x compared with current technology. Implemented as a novel actively cooled catheter of MRI-compatible materials, our approach will produce a versatile intravascular 3D ICE catheter that can operate under conventional X-ray and MRI. The 3D MRICE and custom, GPU based real-time imaging system will be evaluated systematically through in-vitro and in-vivo studies with special emphasis on MRI safety leading to a an Phase I IDE clinical trial at the end of year 3. If successful, our work will address a key unmet need and fundamentally enhance image-guided catheter treatments of complex heart disease to improve effectiveness and outcome.